The conservation and optimal use of energy is a key consideration in the manufacture and running of modern vehicles and machinery. There is an increasing user demand for efficiency and for obtaining the best possible output at the lowest possible cost to the user. Considerations in this cost/output balance include both financial and environmental factors. In addition, there is a demand for improved power and speed from vehicles and machinery, whilst at the same time a desire to provide a comfortable and user friendly feel. Furthermore, there is a trend for engines, motors and other equipment to become ever more compact and streamlined.
There are many known approaches for dealing with the above discussed balance. For example, as user demand for environmentally friendly vehicles grows and the regulations on carbon emissions become ever stricter, hybrid vehicles are becoming more popular. As will be known to the skilled reader, hybrid vehicles use a combination of two or more different power sources to move a vehicle or otherwise power machinery. In the field of motor vehicles, the most common hybrid is a hybrid electric vehicle (HEV) which combines an internal combustion engine (ICE) with one or more electric motors. Depending on the power demand at any given time, one or both of the ICE and the electric motor will be deployed to provide power to the vehicle's outputs. A chemical energy storage system is provided in conjunction with the electric motor so that, during periods when the electric motor is not being used to power vehicle output, it can instead operate as a generator to create and store charge in the chemical energy storage system for later use. Known chemical energy storage systems can be made up of a single type of chemical cell or can comprise any combination of cells having differing chemical formulations. All such chemical energy storage systems are designated herein as being a chemical “battery”.
Problems exist with known hybrid systems since the high cycling frequency of the hybrid battery system charge level caused by, for example, regenerative braking and recovery during a typical vehicle usage scenario and the high power flows associated with these operations accelerate the deterioration of battery health, thereby limiting the system life. Warranty is thus often limited on batteries in conventional hybrid systems. Typically, the chemical battery may have to be changed twice during the lifetime of known hybrid electric vehicles. Furthermore, battery cycling may be limited by protective control systems which control distribution of power supply and/or charging in a hybrid system. An effect of this protective limiting is to impair the CO2 reduction efficacy of the corresponding hybrid system.
Flywheels are known for the storage of energy in the form of kinetic energy, for example for use in vehicles. It is known to use a flywheel to store the energy which would otherwise be converted to heat in a vehicle's braking system when the vehicle decelerates, this stored energy then being available for use to accelerate the vehicle when desired. However, a problem with existing flywheel implementations remains that of how to charge the flywheel initially and at points of low energy therein.
Another problem with existing flywheel implementations is that when, for example, a vehicle is brought to rest and the engine (e.g. an Internal Combustion Engine or ICE) is switched off, the flywheel remains rotating and charged with kinetic energy. This kinetic energy is gradually lost as the flywheel rotation speed gradually decreases due to friction, thus energy recovery efficiency is compromised.
A further problem with existing flywheel implementations is that under regenerative conditions, e.g when a vehicle is slowing down, if the flywheel is already rotating at or near maximum speed then further charging of the flywheel can result in dangerous operating speeds, and possibly failure of the flywheel. A way of avoiding this possibility without wasting potentially recoverable energy is therefore desirable.
A further problem with existing flywheel implementations is that after a sustained acceleration event, most of the flywheel's energy will have been used to aid acceleration, and the flywheel speed will be consequently reduced to at or near its minimum speed. If a subsequent acceleration event occurs before a significant regeneration event occurs, the flywheel will contain little energy for use in aiding the subsequent acceleration. Thus, the subsequent acceleration attempt will suffer from stunted performance compared to the first acceleration event (when the flywheel was in a relatively fully charged state). Such unpredictability of performance is undesirable and could even be dangerous, for example when an overtaking manoeuvre is being attempted.
A further problem with some existing flywheel implementations is that stored flywheel energy is dissipated over time due to frictional losses. A way of reducing such losses is desirable.
As discussed above, in order to optimally balance cost and output in a vehicle or machine it is desirable to harness as much of the available energy as possible and to prevent energy merely being dissipated as, for example, heat energy. Hence there is an ongoing requirement for apparatus and methods that optimise use of energy in vehicles and other machinery whilst at the same time not compromising on user-important factors such as comfort, cost effectiveness and environmental friendliness.
An invention is set out in the claims.